Use of magnetic media for mass storage of digital data in a computer system is widespread. Digital data is generally stored on a magnetic medium in the form of magnetic polarity inversions induced into the surface of the medium. If the medium is a magnetic disk, for example, the data is usually arranged in a series of concentric annuluses on the disk's surface, known as tracks. To read data from one of these tracks, the disk is rotated at a constant speed, and a magnetic transducer is brought near the rotating track to convert the alternating magnetic field emanating from the track surface into an analog electrical signal. One type of magnetic transducer which is widely used for reading digital data from a magnetic medium is a magnetoresistive (MR) head.
An MR head is a device whose resistance varies with the applied magnetic field. In this regard, the head is capable of converting magnetic field variations produced by a rotating track into a time varying voltage or current in an electrical circuit. MR heads offer many advantages over other types of magnetic transducers and, consequently, are increasingly being used in magnetic data storage systems. For example, MR heads are more sensitive than other types of read heads, such as thin film heads, and produce a stronger read signal. Also, MR heads have a better frequency response than other types of heads which use inductive coils as a sensing means. In addition, the read signal produced by an MR head is relatively insensitive to the relative velocity between the head and the medium, as is the case with other types of heads, because it is the level of the applied magnetic field which is sensed by an MR head and not the rate of change of magnetic flux lines through a coil. This is an advantage in systems where head/medium velocity may vary over a significant range. Lastly, because MR heads are not capable of writing data on a magnetic medium, magnetic data storage systems which use MR read heads must include a separate head to perform the write function. Using a separate head for reading and writing allows each head to be separately optimized for performing its singular task which can greatly improve the performance of a magnetic data storage system.
As illustrated in the characteristic of FIG. 1, the relationship between theiresistance of an MR head and the applied magnetic field is nonlinear. This nonlinear characteristic can produce problems in the conversion of the magnetic field variations emanating from the medium into the time varying electrical signal. For example, the nonlinear nature of the MR head may cause the time varying electrical signal produced by the head to look nothing like the magnetic signal applied to the head. To overcome this problem, a bias current is generally applied to the head to move the quiescent operating point of the head to a more linear region of the resistance characteristic. With reference to FIG. 1, it is seen that maximum linearity in the operation of an MR head is obtained by biasing the head at point A, i.e., the most linear point on the characteristic. It may be desirable, however, to bias the head at another point, such as point B or point C, to maximize a conversion parameter which may be more important than linearity, such as signal to noise ratio (SNR). As a consequence of such biasing, the output signal of the head may be asymmetrical about a zero volt baseline, such as output waveform 10 in FIG. 1 corresponding to bias point B. In addition to biasing effects, other factors may also exist which result in an asymmetrical read signal, such as off-track effects.
Because of the high data densities being stored on magnetic media today, read signals are comprised of relatively narrow electrical pulses and read signal asymmetry can make detection of the stored data bits difficult. For example, in a disk drive using a peak detector, the difference in the magnitude of the positive and negative peaks of the read signal complicates, among other things, the setting of the threshold levels used in detecting the peaks. Alternatively, in a disk drive using a partial response maximum likelihood (PRML) channel, the difference in the magnitudes of the positive and negative peaks of the read signal complicates the sampling of the signal which must be performed before maximum-likelihood detection can occur. A need therefore exists for an apparatus which is capable of overcoming the problems created by an asymmetrical read signal produced by an MR head.
In addition to the above-described asymmetry, MR heads are also known to produce a shift in the baseline of the signal read from the magnetic medium. FIG. 2 illustrates a read signal having such a baseline shift 12. It has been proposed that this baseline shift 12 is caused by the presence of parasitic magnetic dipoles along the edges of the data track which are sensed by the MR head during readback. An expanded discussion of this phenomena can be found in "Track Edge Phenomena in Thin Film Longitudinal Media," IEEE Transactions on Magnetics, Vol. 25, No. 5, September 1989, by Su et al. In addition, shifted baselines may also be present in read signals produced by other types of heads, such as thin film heads. As with asymmetrical peaks, the presence of shifted baselines can complicate the detection of the data stored on the magnetic medium. A need therefore exists for an apparatus which is capable of overcoming the problems created by the baseline shift of the read signal.